U.S. Pat. Nos. 5,185,636, 5,283,628, and 5,309,221, assigned to the assignee of this application, describe techniques for monitoring various properties of an optical waveguide fiber during the drawing of the fiber from a preform.
In particular, U.S. Pat. No. 5,309,221 is directed to measuring the diameter of a fiber with high precision, U.S. Pat. No. 5,283,628 deals with measuring the diameter of a non-circular fiber and provides a method for characterizing the extent to which a fiber is noncircular, and U.S. Pat. No. 5,185,636 describes techniques for detecting defects in fibers.
In their preferred embodiments, each of these patents employs the fiber monitoring system shown schematically in FIGS. 1 and 2. FIG. 1 shows the basic elements of an interference-based system for measuring fiber diameters as originally disclosed in U.S. Pat. Nos. 3,982,816 and 4,067,651 to Lawrence Watkins. See also Murphy et al., U.S. Pat. No. 4,280,827.
As shown in this figure, an optical waveguide fiber 13, whose cross-section has been greatly expanded for purposes of illustration, is transversely illuminated by light 25 of sufficient spatial coherence and monochromaticity to create a discernible interference pattern in the far field. The interference pattern arises from the superposition of light reflected from the fiber surface 17 and light refracted through the fiber body 13. In practice, as shown in FIG. 2, a laser 23, e.g., a helium-neon (HeNe) laser, is the preferred light source because of its wavelength stability. The following discussion is thus in terms of a laser light source, it being understood that other light sources having sufficient spatial coherence and monochromaticity can be used if desired.
As explained in the above-referenced Watkins patents, in the far field, the light reflected and refracted from fiber 13 interferes to form fringe pattern 19. For an optical waveguide fiber having a core and a cladding, the fringe pattern will in general be a function of the wavelength of the incident light and of the indices of refraction and the diameters of both the core and the cladding. However, as shown by Watkins, if the core/clad ratio is not too large and if the fringe pattern is examined at sufficiently large angles, above about .+-.50.degree. in FIG. 1 for core/clad ratios of less than about 0.5, the pattern will depend almost exclusively on the diameter and index of refraction of the cladding.
Accordingly, if the index of refraction of the cladding is known, the outside diameter (O.D.) of the fiber can be determined by analyzing the fringe pattern. For example, the diameter can be estimated with relatively good precision by counting the number of full and partial fringes between two angles and then converting that number to a fiber diameter value using the equations of the Watkins patents or an empirical calibration.
Rather than counting fringes, fiber diameters can also be determined by generating a spatial frequency spectrum for the interference pattern and detecting a component of that spectrum which corresponds to the outside diameter of the fiber (the "outside diameter" or "O.D." component). Like the number of fringes between two angles, the frequency of the O.D. component is directly related to the diameter of the fiber.
The use of spatial frequency spectra to measure fiber diameters is discussed in an article by Mustafa A. G. Abushagur and Nicholas George entitled "Measurement of optical fiber diameter using the fast Fourier transform," Applied Optics, Vol. 19, pages 2031-2033 (1980). In particular, this article discusses the use of fast Fourier transforms (FFTs) to generate the frequency spectrum from which the O.D. component is detected.
U.S. Pat. No. 5,309,221, referred to above, describes improved methods for determining the frequency of the O.D. component. The methods for identifying the O.D. component disclosed in that patent are the preferred methods for practicing certain embodiments of the present invention (see below).
U.S. Pat. No. 5,283,628, also referred to above, extends the Watkins approach to non-circular fibers. Preferred equipment for doing so is shown in FIG. 2 where optical systems 26,27 project the far-field interference pattern 19 of FIG. 1 onto left and right detectors 29,31. The detectors can comprise linear arrays of photodetectors whose output, after analog to digital conversion, consists of a digital representation of the fringe pattern. A discussion of optical systems 26,27 and their relationship to detectors 29,31 can be found in the above referenced U.S. Pat. No. 5,309,221.
As shown in FIG. 2, the center of detector 29 lies at +61.5.degree. and the center of detector 31 lies at -61.5.degree.. The derivation of these values is discussed in detail in the above referenced U.S. Pat. No. 5,283,628. A preferred angular extent for each detector is 16.degree., i.e., from +53.5.degree. to +69.5.degree. for detector 29 and from -53.5.degree. to -69.5.degree. for detector 31. Detectors having other angular extents can, of course, be used if desired.
In accordance with U.S. Pat. No. 5,283,628, the output of each detector is analyzed separately to generate a signal representative of the diameter of the fiber, and the two signals are averaged to produce a final signal which is representative of the fiber diameter and which is substantially insensitive to fiber ellipticity.
U.S. Pat. No. 5,185,636, referred to above, further extends the Watkins approach to the detection of defects in fibers. FIG. 3 hereof is a copy of FIG. 7(b) of U.S. Pat. No. 5,185,636, and constitutes a spatial frequency spectrum for a computed far-field interference pattern of a 125 micron, coreless fiber containing a 5 micron on-center hole for the angular range between 50 and 70 degrees in FIG. 1 hereof. The O.D. component for this fiber occurs at a spatial frequency of about 3.1 fringes/degree. The second peak in this spectrum, i.e., the one occurring at about 1.7 fringes/degree, is produced by the 5 micron hole (defect). As explained in U.S. Pat. No. 5,185,636, by searching for such second peaks, the presence of defects in a fiber can be readily determined.
For ease of reference, a second peak of the type shown in FIG. 3 will be referred to herein as a "defect component" of a fiber's spatial frequency pattern. It should be noted that there is a third peak in FIG. 3 at around zero fringes/degree. This peak will be referred to herein as the "DC component".
EPO Patent Publication No. 608,538, also assigned to the assignee of the present invention, discloses a further use for the spatial frequency spectrum of a fiber. The problem which this patent publication addresses is that of monitoring the thickness of hermetic coatings, e.g., carbon coatings, which are applied to fibers during the drawing process to reduce, among other things, water corrosion. As explained therein, the magnitude of the O.D. component is inversely related to the thickness of the hermetic coating and thus this magnitude (and preferably this magnitude normalized by the magnitude of the D.C. component) can be effectively used to monitor and control the application of such coatings. When a spatial frequency spectrum is used for this purpose, the laser light applied to the fiber to generate the Watkins interference pattern is preferably polarized so that the beam's electric field component is substantially parallel to the longitudinal axis of the fiber (referred to hereinafter as "polarized illumination").
The foregoing, commonly assigned, U.S. Pat. Nos. 5,185,636, 5,283,628, and 5,309,221 and EPO Patent Publication No. 608,538 are hereby incorporated herein by reference.